The invention is especially, but not exclusively, applicable to the formation of a layer of compacted powdery microporous thermal insulation material in a supporting dish for a radiant electric heater such as is used in smooth top glass ceramic cooking appliances.
The term `microporous` is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller. A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is collisions between air molecules). Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. Other equivalent open lattice structures include pyrogenic (fumed) and electrothermal types in which a substantial proportion of the particles have an ultimate particle size less than 100 nm. Any of these materials, based for example on silica, alumina or other metal oxides, may be used to prepare a composition which is microporous as defined above.
Known forms of microporous materials comprise compacted microporous metal oxide particles, particularly silica, and typically including ceramic fibre or glass filament reinforcement and rutile powder opacifier. Alumina is sometimes included to improve high temperature performance by reducing shrinkage, this being particularly important in radiant electric heater applications.